This invention relates to a windshield wiper system and, more particularly, to a windshield wiper system which utilizes a reciprocating, flexible arm comprising a for driving a windshield wiper.
An example of a prior art windshield wiper drive link and system is shown in Buchanan et al., U.S. Pat. No. 6,148,470, which is incorporated herein by reference and made a part hereof. A windshield wiper system, as taught therein, is particularly useful for driving in snow or in mud, under conditions wherein an accumulation of foreign material may cause a sudden blockage of the wiper block. When this happens, the windshield wiper motor may generate a momentarily large driving torque in an attempt to overcome the blockage. That in turn may cause permanent damage to one or more components of the wiper system.
A flexible arm, as taught in Buchanan et al., reduces the risk of such damage by constructing the wiper drive arm from a material which tolerates compression loads up to a predetermined limit. Below that limit, known as the critical buckling load limit, the drive arm simply compresses by an amount proportional to the force of the load. However, upon reaching the critical buckling load limit, the arm gives way by pronounced elastic buckling. The buckling effectively prevents any further increase in the load being applied to wiper system components, and does so without permanent injury to the drive arm. Once the blockage has been removed, manually or otherwise, the flexible arm simply pops back into its original configuration.
As further taught in Buchanan et. al. U.S. Pat. No. 6,148,470, the flexible drive arm may be interposed between a drive motor and a pair of drive plates. The drive plates in turn apply drive torques cooperatively to a pair of wiper blades. The flexible drive arm preferably is made from a composite material of a type described in Table I of the patent. Four specific materials are taught, including a molded glass laminate, a molded epoxy resin, and two pull-molded polyesters having oriented glass fibers.
As further disclosed in Buchanan et al, the flexible drive arm may be generally elongated and generally rectangular in cross-section. The patent teaches that the flexible drive arm could have other cross-sectional geometries, such as elliptical or circular, and in one described configuration could have a length of at least about 250 mm. Notches could be fabricated in the flexible drive arm in order to adjust the bending stress at which elastic buckling occurs. The patent observes that a suitable flexible drive arm should have a design strength such that buckling is not expected to occur in the face of a compression load less than about 30 percent greater than the normally expected maximum running load for a comparably sized steel or rigid link that does not flex.
The prior art also includes a windshield wiper for an aircraft, as shown, for example in Rogers et. al (U.S. Pat. No. 4,318,201). That patent teaches a flexible drive arm for a windshield wiper wherein the cross-section varies from end to end in order to control the onset of elastic buckling. The Rogers patent also discloses the use of a glass fiber composite for construction of a flexible drive arm for a windshield wiper.
This invention improves the performance of a windshield wiper by providing it with a flexible drive arm supported by a hollow tube extending from a motor to a wiper arm. The tube preferably has a normally unstressed sideward curvature for relaxation along a windshield when the wiper axis rests on a curved portion thereof. The cross-section of the hollow tube has an off-center shear center. As the wiper axis moves to a flat portion of the windshield, the contact of the wiper against the windshield generates a sidewardly directed bending (unbending) force which stresses and straightens out the drive arm along a cross-sectional width. The straightening of the drive arm sets up internal bending stresses which flatten the hollow tube thereby progressively decreasing the moment of inertia about the longitudinal axis. Transverse blocking results in elastic buckling when the sidewardly applied bending force reaches a predetermined level. That in turn relieves the stress on the windshield drive motor and wiper components when the wiper system or arm becomes blocked.
In one aspect, this invention comprises a drive arm for a windshield wiper comprising a tubular member having a preselected cross-section and an internal cavity extending initially forward along a longitudinal axis, with a curvature extending laterally from said longitudinal axis; and said tubular member comprising a wiper axis for connection of a wiper thereto and a motor axis for connection of a drive motor thereto; and said wiper axis being selected to cause a bending of said tubular member about said longitudinal axis when a bending force is applied thereto.
In another aspect, this invention comprises a windshield wiper system comprising a drive motor, a drive arm coupled to said drive motor; and a wiper blade also coupled to said drive arm for wiping a windshield when said drive motor is energized; said drive arm being made of a composite material and being generally curved in cross-section and an internal cavity extending along its length.
Other objects and advantages of the invention will be apparent from the following description, the accompanying drawings and the appended claims.